Luke 1:1-4

1Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us, 2Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the word; 3It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus, 4That thou mightest know the certainty of those things, wherein thou hast been instructed.

Comment: Luke, at the beginning of his gospel, indicates there were eyewitnesses besides the apostles to the events in Jesus' life. Luke also quotes Paul and Barnabas as making this point in Acts 13:30-31 and indirectly includes it in Acts 1:21-26. Luke states that he had personally verified the accuracy of these eyewitness accounts. This means every event recorded in the gospel of Luke had been investigated including Jesus' resurrection. For example, Luke would've had to check public records for things like births, accounts of Jesus crucifixion by the Roman authorities, and John the Baptist's ministry and death. Also he would have had to interview some of the eyewitnesses. Luke would have necessarily needed to perform this sort of verification since he had not become a Christian, in all likelihood, until Paul's second missionary journey [51-54 AD]. In fact, Luke appeals to the fact that he had gone to all this trouble as the basis for believing what he had managed to summarize in his gospel. This indicates Luke wasn't just someone looking to join a new religion, but someone who needed to be convinced before believing what he was told. And finally, how could Luke honestly claim to be reassuring Theophilus of the certainty of what Theophilus had been taught unless he had actually investigated the events?